Microparticles, microcapsules and microspheres (hereinafter "microparticles") have important applications in the pharmaceutical, agricultural, textile and cosmetics industry as delivery vehicles. In these fields of application, a drug, protein, hormone, peptide, fertilizer, pesticide, herbicide, dye, fragrance or other agent is encapsulated in a polymer matrix and delivered to a site either instantaneously or in a controlled manner in response to some external impetus (i.e., pH, heat, water, radiation, pressure, concentration gradients, etc.). Microparticle size can be an important factor in determining the release rate of the encapsulated material.
Many microencapsulation techniques exist which can produce a variety of particle types and sizes under various conditions. Methods typically involve solidifying emulsified liquid polymer droplets by changing temperature, evaporating solvent, or adding chemical cross-linking agents. Physical and chemical properties of the encapsulant and the material to be encapsulated can sometimes dictate the suitable methods of encapsulation, making only certain methodologies useful in certain circumstances. Factors such as hydrophobicity, molecular weight, chemical stability, and thermal stability affect encapsulation. Significant losses are frequently associated with multiple processing steps. These parameters can be particularly important in respect of encapsulating bioactive agents because losses in the bioactivity of the material due to the processing steps or low yields can be extremely undesirable.
Common microencapsulation techniques include interfacial polycondensation, spray drying, hot melt microencapsulation, and phase separation techniques (solvent removal and solvent evaporation). Interfacial polycondensation can be used to microencapsulate a core material in the following manner. One monomer and the core material are dissolved in a solvent. A second monomer is dissolved in a second solvent (typically aqueous) which is immiscible with the first. An emulsion is formed by suspending the first solution through stirring in the second solution. Once the emulsion is stabilized, an initiator is added to the aqueous phase causing interfacial polymerization at the interface of each droplet of emulsion.
Spray drying is typically a process for preparing 1-10 micron sized microspheres in which the core material to be encapsulated is dispersed or dissolved in a polymer solution (typically aqueous), the solution or dispersion is pumped through a micronizing nozzle driven by a flow of compressed gas, and the resulting aerosol is suspended in a heated cyclone of air, allowing the solvent to evaporate from the microdroplets. The solidified particles pass into a second chamber and are trapped in a collection flask. This process can result in 50-80% loss through the exhaust vent when laboratory scale spray dryers are used.
Hot melt microencapsulation is a method in which a core material is added to molten polymer. This mixture is suspended as molten droplets in a nonsolvent for the polymer (often oil-based) which has been heated to .apprxeq.10.degree. C. above the melting point of the polymer. The emulsion is maintained through vigorous stirring while the nonsolvent bath is quickly cooled below the glass transition of the polymer, causing the molten droplets to solidify and entrap the core material. Microspheres produced by this technique typically range in size from 50 microns to 2 mm in diameter. This process requires the use of polymers with fairly low melting temperatures (i.e., &lt;150.degree. C.), glass transition temperatures above room temperature, and core materials which are thermo-stable.
In solvent evaporation microencapsulation, the polymer is typically dissolved in a water immiscible organic solvent and the material to be encapsulated is added to the polymer solution as a suspension or solution in organic solvent. An emulsion is formed by adding this suspension or solution to a beaker of vigorously stirring water (often containing a surface active agent to stabilize the emulsion). The organic solvent is evaporated while continuing to stir. Evaporation results in precipitation of the polymer, forming solid microcapsules containing core material.
A solvent evaporation process exists which is specifically designed to entrap a liquid core material in PLA, PLA/PGA copolymer, or PLA/PCL copolymer microcapsules. The PLA or copolymer is dissolved in a miscible mixture of solvent and nonsolvent, at a nonsolvent concentration which is immediately below the concentration which would produce phase separation (i.e., cloud point). The liquid core material is added to the solution while agitating to form an emulsion and disperse the material as droplets. Solvent and nonsolvent are vaporized, with the solvent being vaporized at a faster rate, causing the PLA or copolymer to phase separate and migrate towards the surface of the core material droplets. This phase separated solution is then transferred into an agitated volume of nonsolvent, causing any remaining dissolved PLA or copolymer to precipitate and extracting any residual solvent from the formed membrane. The result is a microcapsule composed of PLA or copolymer shell with a core of liquid material.
In solvent removal microencapsulation, the polymer is typically dissolved in an oil miscible organic solvent and the material to be encapsulated is added to the polymer solution as a suspension or solution in organic solvent. An emulsion is formed by adding this suspension or solution to a beaker of vigorously stirring oil, in which the oil is a nonsolvent for the polymer and the polymer/solvent solution is immiscible in the oil. The organic solvent is removed by diffusion into the oil phase while continuing to stir. Solvent removal results in precipitation of the polymer, forming solid microcapsules containing core material.
Phase separation microencapsulation is typically performed by dispersing the material to be encapsulated in a polymer solution by stirring. While continuing to uniformly suspend the material through stirring, a nonsolvent for the polymer is slowly added to the solution to decrease the polymer's solubility. Depending on the solubility of the polymer in the solvent and nonsolvent, the polymer either precipitates or phase separates into a polymer rich and a polymer poor phase. Under proper conditions, the polymer in the polymer rich phase will migrate to the interface with the continuous phase, encapsulating the core material in a droplet with an outer polymer shell.
A recent patent to Tice (U.S. Pat. No. 5,407,609) involves a phase separation microencapsulation process which attempts to proceed more rapidly than the procedure described in the preceding paragraph. According to Tice, a polymer is dissolved in the solvent. An agent to be encapsulated then is dissolved or dispersed in that solvent. The mixture then is combined with an excess of nonsolvent and is emulsified and stabilized, whereby the polymer solvent no longer is the continuous phase. Aggressive emulsification conditions are applied in order to produce microdroplets of the polymer solvent. After emulsification, the stable emulsion is introduced into a large volume of nonsolvent to extract the polymer solvent and form microparticles. The size of the microparticles is determined by the size of the microdroplets of polymer solvent. This procedure has the drawback that small particles can be obtained only with aggressive emulsification procedures. It also suffers the drawback that multiple processing steps are required to form the microparticles.
Phase inversion is a term used to describe the physical phenomena by which a polymer dissolved in a continuous phase solvent system inverts into a solid macromolecular network in which the polymer is the continuous phase. This event can be induced through several means: removal of solvent (e.g., evaporation; also known as dry process), addition of another species, addition of a non-solvent or addition to a non-solvent (also known as wet process). In the wet process, the polymer solution can be poured or extruded into a non-solvent bath. The process proceeds in the following manner. The polymer solution undergoes a transition from a single phase homogeneous solution to an unstable two phase mixture:polymer rich and polymer poor fractions. Micellar droplets of nonsolvent in the polymer rich phase serve as nucleation sites and become coated with polymer. At a critical concentration of polymer, the droplets precipitate from solution and solidify. Given favorable surface energy, viscosity and polymer concentrations, the micelles coalesce and precipitate to form a continuous polymer network.
Phase inversion phenomenon have been applied to produce macro and microporous polymer membranes and hollow fibers used in gas separation, ultrafiltration, ion exchange, and reverse osmosis. Structural integrity and morphological properties of these membranes are functions of polymer molecular weight, polymer concentration, solution viscosity, temperature and solubility parameters (of polymer, solvent and non-solvent). For wet process phase inversion, polymer viscosities must be greater than approximately 10,000 centipoise to maintain membrane integrity; lower viscosity solutions may produce fragmented polymer particles as opposed to a continuous system. Furthermore, it is known that the quicker a solution is caused to precipitate, the finer is the dispersion of the precipitating phase.
A phase inversion process has been employed to produce polymer microcapsules. The microcapsules are prepared by dissolving a polymer in an organic solvent, forming droplets of the solution by forcing it through a spinneret or syringe needle, (the size of which droplets determines the size of the final microcapsule), and contacting the droplets with a nonsolvent for the polymer which is highly miscible with the polymer solvent, thereby causing rapid precipitation of the outer layer of the droplet. The microcapsules must be left in contact with the nonsolvent until substantially all of the solvent has been replaced with nonsolvent. This process requires formation of a droplet with dimensions established prior to contacting the nonsolvent.
Each of the methods described before require the formation of an emulsion or droplets prior to precipitation of the final microparticle. The present invention provides a novel method of producing microparticles without the requirement of forming an emulsion prior to precipitation. Under proper conditions, polymer solutions can be forced to phase invert into fragmented spherical polymer particles when added to appropriate nonsolvents. We have utilized this spontaneous microparticle formation phase inversion process as a rapid, one step microencapsulation technique. The process is simple to perform, is suitable with a number of polymeric systems (including many common degradable and non-degradable polymers typically employed as controlled release systems), produces extremely small microparticles (10 nm to 10 .mu.m) and results in very high yields.